Wet season notes 2001-05

Background descriptions about the nature and sources of mine-related contaminants which could reach Alligator Rivers Region streams from the Ranger and Jabiluka mine sites may be found in the 'Background Paper' on environmental monitoring.

Data are provided for three streams on ERA leases. These streams are Magela Creek, which flows immediately east of the Ranger mine, Gulungul Creek, which flows immediately west of Ranger, and Ngarradj (Swift Creek) which flows east of the Jabiluka minesite.

The first water chemistry samples for the Supervising Scientist's 2004-05 wet season surface water monitoring program were collected from the Magela Creek downstream statutory compliance point on 21 December 2004, one day after the commencement of flow past the site . Weekly sampling continued throughout the wet season with the last of the routine monitoring samples collected on 25 May 2005, the week before flow past the downstream compliance point ceased. Investigative monitoring continued for several weeks at other sites in Magela Creek. Results from those investigations will be used to assess aspects of the monitoring program.

All indicators remained within limits/guidelines throughout the 2004-05 wet season and the quality of water was comparable to that seen in previous years.

Uranium, magnesium and sulfate were generally slightly higher downstream of the mine but the concentrations were very low and not of environmental concern. The downstream sulfate concentrations were slightly higher this season than in the last two seasons but were lower than those measured most years since the mid 1980's. Uranium was less than 1% of the limit for most of the season. Manganese and the general parameters - pH, electrical conductivity (EC) and turbidity - were almost identical at upstream and downstream sites for the season, except for a slightly elevated manganese concentration in the first week of monitoring.

Flow rates greater than 500 cubic metres per second, the largest flows recorded in Magela Creek for the season, occurred within hours of sample collection on the 23 March 2005. Turbidity peaked and the dilution effects of the storm-related flows are evident in the downstream magnesium, sulfate and conductivity data that week.

Gradual changes in Magela Creek water quality began in April. By May the signal from the mine became less noticeable with magnesium and sulfate concentrations similar at both sites. Magnesium-sulfate enriched surface water discharges from the mine decreased reducing the concentrations downstream. At the same time magnesium concentrations upstream increased as a result of the progressively increasing influence of magnesium rich groundwater to the diminishing volumes of surface flow. Similar trends have been noted in previous years (for example see Commentary on Magela Creek 2002-03 monitoring data).

The water quality objectives set to protect the aquatic ecosystems downstream of the mine were achieved. Available biological monitoring data (see Creek side monitoring results) also indicate that the environment remained protected throughout the season.

The first water chemistry samples for the Supervising Scientist's 2004-05 wet season surface water monitoring program were collected from Gulungul Creek on 23 December 2004 , the first day of flow in the creek for the wet season . Weekly sampling continued throughout the season with the last of the routine monitoring samples collected on 8 June 2005, during the week in which flow past the downstream compliance point ceased.

Uranium concentrations in Gulungul Creek are slightly higher than those in Magela Creek. Like Magela Creek, the concentrations at the Gulungul Creek downstream site are slightly higher than those at the upstream site. However, the uranium concentrations in Gulungul Creek were well below the limit throughout the season, with the concentration at both the upstream and downstream sites ranging between about 1 and 4% of the limit.

Sulfate concentrations in Gulungul Creek were similar to those seen in previous years, with the downstream concentrations generally higher than those upstream but still well below levels of environmental concern.

The small difference in electrical conductivity (EC) between the upstream and downstream sites shows that the sulfate increase has little effect on the overall solute levels downstream of the mine. The EC trend at both upstream and downstream sites closely follows that of magnesium which is not influenced by the mine, ie concentrations are very similar upstream and downstream of the mine. Manganese, pH and turbidity were also similar at the upstream and downstream sites.

Seasonal effects similar to those described for Magela Creek can also be seen in Gulungul Creek. A gradual increase in magnesium concentrations and EC both upstream and downstream of the mine and a downstream decrease in sulfate began in April. A slightly elevated manganese concentration was measured downstream during the last week of flow. Similar trends in Gulungul Creek have been noted in previous years (for example see Commentary on Gulungul Creek 2002-03 monitoring data).

Toward the end of 2003 Jabiluka entered a long-term care and maintenance phase. The site poses a very low risk to the environment (see Commentary on Ngarradj (Swift Creek) 2003-04 monitoring data). Consequently, the Supervising Scientist's water chemistry monitoring program at Ngarradj was reduced to monthly sampling for the 2004-05 wet season, augmented by a utomatic recordings of turbidity and hydrological data at 6-minute intervals. The NT Department of Business, Industry and Resource Development (DBIRD) resumed the role of performing check monitoring at Ngarradj, also on a monthly basis. These independent programs complement each other, providing fortnightly water sampling and a combined dataset to assess the water quality at Ngarradj. ERA continued to carry out independent monitoring on a weekly basis.

The first water chemistry samples for the Supervising Scientist's 2004-05 wet season surface water monitoring program were collected from the Ngarradj downstream statutory compliance point on 6 January 2005 . ERA commenced monitoring at that site on the 29 December 2004, the day flow was first observed, and DBIRD commenced monitoring in the second week of January. The last samples were collected from Ngarradj on 10 May 2005, by ERA, shortly before flow ceased.

ERA & SSD data, which agree well, show values and trends similar to those seen in previous years. The water quality was very good throughout the season with only one exceedance of the electrical conductivity (EC) and the magnesium guideline. All other key indicators remained within limits/guidelines, with uranium remaining less than 0.5% of the limit.

EC at both sites was low throughout the season except for one occasion in early January (SSD data) when EC at the downstream site was just above the guideline value. However, the corresponding EC at the upstream site was of a similar value, indicating that the elevation was part of a natural fluctuation in the system.

The magnesium concentration exceeded the upper guideline in May just before flow ceased. When the water level in the creek drops toward the end of the season we expect to see increases in magnesium concentrations, particularly at the downstream site, which naturally has higher concentrations than upstream. Therefore, the upper level for magnesium has always been a guideline and not a limit. This trend has been noted in previous years in Ngarradj ( for example see Commentary on Ngarradj (Swift Creek) 2002-03 monitoring data ) and at other creeks in the region (eg Magela and Gulungul Creek).

The water quality objectives set to protect the aquatic ecosystems downstream of Jabiluka were achieved providing assurance that the environment remained protected throughout the season.

The first water chemistry sample for the Supervising Scientist's 2003-04 wet season surface water monitoring program was collected from the Magela Creek downstream statutory compliance point on December 23rd , one day after the commencement of flow past the site for this wet season . Weekly sampling continued throughout the wet season with the last of the routine monitoring samples collected on July 2nd . Further sets of samples were collected in Magela Creek for investigative purposes (rather than for compliance monitoring) on:

March 26th to April 2nd , the week following the Ranger drinking water contamination incident, and

July 10th , by which time the central channel at the downstream monitoring site was a series of disconnected pools with flow at the compliance point almost negligible and lateral from the very shallow (

All indicators measured at the Magela Creek compliance point during flow have been well within limits this season, even during the first weeks when first flush effects often result in limits or guidelines being approached and occasionally exceeded ( see 2002-03 wet season notes ). The final manganese measurement (discussed below), while above the limit, is not considered a breach of compliance, as there was no flow past the compliance point at the time of sampling. Compliance with the water quality limits and guidelines throughout the season provides reassurance that the aquatic environment has not suffered any deleterious effects from mining during the previous year. The results of the biological monitoring program , when available, are expected to confirm this.

Overall, the water quality in Magela Creek this wet season was similar to that seen last season, and was demonstrably improved compared to that seen during the 2001-02 season (note the smaller differences in upstream-downstream electrical conductivity and concentrations of uranium, magnesium and sulfate seen in the last two seasons compared to the 2001-02 season). These improvements are likely due to a range of remedial works and water management practices implemented at the mine in recent years. Values for electrical conductivity (EC), pH, turbidity and manganese concentration were almost identical at both sites throughout most of the season. Magnesium and sulfate values were similar at both sites at the beginning and end of the season, but were higher at the downstream site during the greater part of the wet season proper when waste-water inputs to the creek are received from the mine site via Retention Pond 1, Georgetown Creek and, for a short period of controlled release, Djalkmara Billabong.

Intermittent pumping of Djalkmara Billabong water into Magela Creek began on the 31st of January and ceased in late March. (This pumping is permitted under strict conditions based on results of 'Whole of Effluent' ecotoxicological testing, flow rates in Magela Creek and predictive modelling of expected concentration increases at the downstream Magela Creek statutory compliance point.) During Djalkmara pumping, a slight rise in uranium concentration downstream of the mine was noticeable in the early February data and again in early March - the uranium concentrations did not exceed 2% of the ecotoxicological limit on those occasions. The other downstream parameters showed little, if any, additional change during the pumping of Djalkmara Billabong.

Toward the end of the wet season, pH, electrical conductivity and magnesium concentrations at both the upstream and downstream sites increased. These trends are typical of late wet season flow conditions when water level in the creek begins to reduce; the fact that increases occurred upstream of the site indicates that they are not attributable to mining.

Elevated manganese concentration at the very end of the season was expected (based on previous years results) but was not seen this year while the creek was flowing at the compliance point. This prompted continuation of sampling, despite the lack of flow at the compliance point, to identify the conditions under which these increases occur. The results indicate, as expected, that the manganese increase is brought about by water quality changes related to the flow conditions, ie the low dissolved oxygen in the near stagnant water causes the manganese present to dissolve. While the seasonal increases in manganese levels are not detrimental to the environment (supported by findings of the biological monitoring program), investigations of their occurrence and causes will be ongoing.

The first water chemistry sample for the Supervising Scientist's 2003-04 wet season surface water monitoring program was collected from Gulungul Creek on December 23rd , one day after the commencement of flow past the site for this wet season . Weekly sampling continued throughout the wet season with the last of the routine monitoring samples collected on July 10th when the creek had all but stopped flowing.

In the first three months of the wet season, the uranium concentrations at the Gulungul Creek downstream site were moderately higher than at the upstream site. The difference, which was greater than in previous years, was most noticeable in mid to early January when the downstream uranium concentration trended away from the upstream concentration. From mid-March, the uranium concentrations at the downstream site were almost the same as those measured at the upstream site, though as in previous years the downstream concentrations are slightly higher than the upstream concentrations. The higher uranium concentrations measured at the downstream site, while probably mine related, are not considered to have posed an environmental risk, the values remaining less than five percent of the limit at the most. Nevertheless, an investigation into the cause of the elevated uranium concentrations at the downstream site will be undertaken.

From early April, pH remained steady at the downstream site while increasing slightly at the upstream site. The measured pH values were similar to those observed in previous years at both upstream and downstream sites, with this season's results showing fewer fluctuations over the season. EC and turbidity measurements recorded this wet season are similar to values measured in previous years and are similar at both the upstream and downstream sites for most of the season. As occurred last wet season, EC generally increased from the middle of the season at both sites (compare to 2002-03 data). However, as was the pattern last season, EC exhibited a sharp increase in the last month. The cause of the elevated EC will be investigated. It cannot be attributed to the common mine-related salts magnesium and sulfate as concentrations of those variables at the downstream site were actually lower during this period than in previous weeks.

Although the uranium concentrations downstream of the mine were elevated relative to the upstream concentrations, the increase was very low compared to the limit. Overall, the water quality in Gulungul Creek was good during the 2003-04 wet season providing reassurance that there is a high degree of certainty that the aquatic environment of Gulungul Creek remained protected from impacts of mining. The results of the biological monitoring program, when available, are expected to confirm this.

The first water chemistry sample for the Supervising Scientist's 2003-04 wet season surface water monitoring program was collected from the Ngarradj downstream statutory compliance point on December 23rd. Water samples were collected weekly for the first seven weeks of the wet season. After considering the reduced risk to the environment posed by the site (which is now in a care and maintenance mode) and the high water quality measured in the early part of the season, the sampling of water for chemical analyses was reduced to fortnightly from February 4th. Automatic gauging stations in Ngarradj upstream and downstream of the Jabiluka site continued to record continuous pH, turbidity and hydrological data, while the biological monitoring program did not change from the previous year. Fortnightly water quality sampling ceased in mid May, when the flow at the downstream compliance site was negligible and by which time flow at the road crossing, further downstream, and in the tributaries had ceased; the last sample for the season was collected on April 25th.

The following points summarise the data collected at Ngarradj as part of the routine water chemistry monitoring program:

The on-site activities carried out over the 2003 dry season (returning the stockpiles underground and irrigation of ponded water) do not appear to have affected metal and ion concentrations in Ngarradj during the 2003-04 wet season, with all measured parameters showing (i) similar trends at both upstream and downstream sites, (ii) similar values to those seen in previous years, and (iii) compliance with the guidelines/limits set for environmental protection (apart from the first flush period discussed below).

First flush effects can be seen in the electrical conductivity (EC) and sulfate data, with values similar at both the upstream and downstream sites in the first weeks of flow. EC and sulfate were above the limit at both sites during this time but fell to within limits by early-mid January. All other indicators were within limits and guidelines during this period.

As observed in previous years, EC and sulfate concentrations were lower downstream than upstream while magnesium concentrations and pH were higher at the downstream site throughout the season. Although higher at the downstream site, both pH and magnesium remained within the guideline range throughout the season and exhibited seasonal trends similar to those observed in previous years; most noticeable was the now-familiar sharp late-season increase in magnesium concentrations that may be attributed to groundwater inputs (compare to 2001-02 and 2002-03 data). pH generally increased at both the upstream and downstream sites from early February (compare to 2001-02 data).

The concentrations of uranium at the downstream site, less than 0.4% of the limit, were similar to the concentrations measured concurrently at the upstream site, and at both sites in previous years (compare to 2001-02 and 2002-03 data).

While no change is obvious in the turbidity measurements recorded during fortnightly sample collection, turbidity changes were measured by another eriss program sampling at different times and under different hydrological conditions. The results of that study, along with more interpretation (after further analyses) of the water chemistry results presented here, will be published in the Supervising Scientist's Annual Report and a Supervising Scientist Series Report later this year.

The only time any parameters measured were above the limits was at the start of the wet season when EC and sulfate were high at both the upstream and downstream sites. As the downstream measurements were almost identical to those upstream, the limit exceedance was obviously not related to mining. The water quality limits and guidelines for Ngarradj were met through the remainder of the season providing reassurance that the aquatic environment did not suffer deleterious effects from activities at the Jabiluka site. The results of the biological monitoring program, when available, are expected to confirm this.

Flow commenced on the 1st of December 2002 in Magela Creek and ceased in the downstream central channel (the downstream monitoring site) in late May 2003. The last monitoring sample collected from that site was on May 22nd. The water quality data collected during the 2002-2003 wet season are described below.

Water quality data for the first few weeks show typical first flush effects at both upstream and downstream sites. Natural first flush waters are typified by relatively low pH and higher electrical conductivity (EC) and metal and ion concentrations than occur during the main period of wet season flow. This is due to flushing of the soil profile and resuspending of billabong and creek bed sediments. Thus during this period pH at both upstream and downstream sites followed similar trends; exceeding the lower guideline twice, while metal and sulfate concentrations and EC were higher at both upstream and downstream sites compared to mid-season values. The manganese concentrations at the downstream site were noticeably higher than the upstream site but remained below the limit. By late December first flush effects ceased and metal and sulfate concentrations and EC fell at both upstream and downstream sites.

Magnesium and sulfate values increased downstream in mid January for a short period. Elevated concentrations of both these ions occurred again towards the end of March at the downstream site. These increased concentrations are associated with dispersion of mine waste waters, mainly from Ranger Retention Pond 1. However, magnesium and sulfate concentrations are interpreted with reference to EC (a surrogate indicator), and these levels remained well below the limit for the whole wet season, with similar values reported at both upstream and downstream sites. Magnesium concentrations at both the upstream and downstream sites increased toward the end of the season as the surface flow receded.

Uranium concentrations at the downstream site rose slightly during active release of Djalkmarra Billabong water to Magela Creek (intermittent during mid February - mid March). Release of water from Djalkmarra is permitted under strict conditions determined primarily by biological toxicity testing. Both ERA and SSD increased the frequency of their water quality monitoring for the release period. SSD also monitored at extra sites in Magela Creek during the release to determine the variation in concentrations across the creek at the downstream compliance point, and conducted routine biological creekside monitoring. SSD monitoring data for the release period show that uranium remained within the natural range of concentrations and was always below 2% of the allowable limit. After the releases ceased, uranium levels at the downstream site quickly returned to levels similar to the upstream site (i.e. ~ 0.5% of the limit).

Throughout the season, EC, turbidity and pH fluctuated but were almost identical at upstream and downstream sites.

As well as first flush, low pH and elevated levels of metals and ions are also common in the late stages of the wet season as a consequence of the increasing contribution of higher-concentration groundwaters as surface flows are declining (see 2001-2002 data and comments). Concentrations of manganese increased sharply at the downstream site in the last few weeks of flow coupled to a sudden decrease in pH (manganese solubility is very responsive to pH changes), with the last pH measurement of the season exceeding the lower guideline. Such natural excursions mean that recommended pH values are guideline values rather than strict limits. Exceeding a guideline value does not imply that the environmental standard has been breached.

In summary, the data show that the Magela Creek water was very high quality throughout the 2002-2003 wet season. Although a signal from the mine can be seen at the downstream site, i.e. elevated uranium during active release and periods of elevated sulfate, the concentrations of these constituents remained very low, with uranium remaining below 2% of the limit and EC remaining at about 50% or less of the limit. Natural seasonal patterns of slightly elevated metals and ions coupled with low pH were seen, as expected, at the start and end of the season (for reasons explained above) - pH exceeded the lower guideline and manganese approached, but did not exceed, the limit during these times. Apart from the seasonal manganese elevations, the values of all measured metals and ions were well below the limits throughout the season indicating that there is a high degree of certainty that the Magela Creek aquatic environment remained protected from mining impacts throughout 2002-2003 wet season. Results from the biological monitoring program verify this.

Gulungul Creek commenced flowing on the 31st of December 2002 and ceased flowing at the downstream monitoring point in early June 2003. The last monitoring sample collected from that site was on May 27th. The water quality data collected during the 2002-2003 wet season are described below.

First flush effects are noticeable in the uranium data, which shows a general decrease in uranium concentrations at both sites throughout the wet season. EC, pH, turbidity and uranium data followed similar trends at both upstream and downstream sites throughout most of the wet season. Uranium concentrations were slightly higher at the downstream site on most occasions but remained well below (limit. On one occasion in late February, a natural increase in uranium concentration at the upstream control site coupled to increased turbidity and EC were observed. This is a good example of how a local storm event can affect water quality by washing soil and organic matter into the creek and stirring up the creek bed sediments. These effects were very localised; the same phenomenon was not seen at the downstream site.

Electrical conductivity (EC) began increasing in mid-March at both upstream and downstream sites. Upstream EC decreased toward the end of the wet season while downstream EC remained elevated. Elevated EC is a typical late season effect that manifests as volumes of surface water diminish toward the end of the wet season and groundwater inputs to the system become more noticeable and/or dissolved salts in the surface water become concentrated as surface water evaporates. The uranium concentrations at the downstream site also rose slightly at the end of the season, probably due to same seasonal effects or to the slight increase in turbidity.

In summary, while it is possible that the higher uranium concentrations at the downstream site were caused by mining, the concentration increases were very low compared to the limit. Hence there is no concern the environment was adversely affected. Further, no limits were exceeded in Gulungul Creek during the 2002-2003 wet season and the chemical data indicate that there is a high degree of certainty that the aquatic environment of Gulungul Creek remained protected from impacts of mining. Results from the biological monitoring program verify this.

Flow past the Ngarradj main channel downstream monitoring site commenced on the 22nd of December 2002 and ceased in mid May 2003. The last monitoring sample was collected on May 13th. The water quality data collected during the 2002-2003 wet season are described below.

First flush effects can be seen in the electrical conductivity (EC) data, which was above the limit at both sites in the first two weeks of flow then fell to within limits in early January. EC remained below the limit throughout the remainder of the wet season, with similar trends observed at both upstream and downstream sites.

Throughout the season, turbidity levels followed similar trends at both sites and, apart from the first upstream measurement, pH remained within the recommended guidelines. Uranium concentrations, which are extremely low (near the limit of detection), were slightly higher at the downstream site but remained well within the limits set and followed similar trends at both sites. The slightly higher uranium concentrations at the downstream site are apparently natural; the same pattern is seen in historical data collected prior to development activities at Jabiluka. For most of the season similar trends were seen at both sites for sulfate and magnesium, though concentrations of sulfate were lower and magnesium higher at the downstream site. These are natural patterns that have also been observed in previous years (see 2001-2002 data and comments).

Magnesium increased suddenly at the downstream site toward the end of the season and exceeded the guideline value. There is a natural source of magnesium present between the upstream and downstream monitoring sites that enriches the groundwater with this ion. With diminishing surface flow, the groundwater entering the creeks makes up a higher proportion of the flow and therefore has a greater impact on the surface water quality. Hence, elevated magnesium at the downstream site is expected during the final stages of flow (see 2001-2002 data and comments). The effect of higher groundwater contribution can also be seen in the increasing EC levels at both sites in the late stage of the wet season.

Because elevated magnesium is natural at the downstream Ngarradj Creek site no limit can be set, only a guideline value. Thus exceeding this value does not imply that the environmental standard has been breached.

In summary, uranium, sulfate, turbidity and EC trends and values were almost identical at the upstream and downstream sites. Uranium concentrations remained extremely low (less than 1% of the limit) at both sites throughout the season. Natural seasonal patterns can be seen in the data, i.e., elevated magnesium concentrations and EC values at the beginning and end of the wet season, elevated uranium concentration and sulfate values during the first flush event with values decreasing throughout the season, and low pH values at the beginning of the season during the first flush period. The pH and magnesium guidelines were exceeded once each. Both incidences were not unexpected, fitting the natural seasonal pattern, and neither of the exceedances were considered to be a breach of environmental standards.

As no limits were exceeded at any time during the season there is a high degree of certainty that the aquatic environment of Ngarradj Creek remained protected from mining impacts throughout the 2002-2003 wet season. Results from the biological monitoring program verify this. Furthermore, the data trends amongst years (including data collected prior to the commencement of mine workings) are almost identical for all parameters, and between-site trends this year are also almost identical for most parameters (magnesium and pH are the exceptions). Therefore, no mine-related signal can be inferred from the 2002-2003 data.

Data for a number of variables for the monitoring period show strong concordance between upstream and downstream values (pH, turbidity) indicating no detectable mine-related effects. These variables are not significantly enhanced or altered in mine waste waters compared with levels found naturally in Magela Creek.

The most evident difference between the upstream and downstream sites for Magela Creek is the presence of elevated concentrations of uranium, magnesium, and sulfate at the downstream location. These differences are almost entirely due to the contribution from Retention Pond 1 (RP1) mine waste-water overflow to Coonjimba Billabong and thence to Magela Creek. When RP1 ceases to overflow, indicator values at the paired sites converge. Relatively minor contributions from Georgetown Billabong, via Georgetown Creek and Corridor Creek, Djalkmara Billabong releases and washoff from the Magela land application area are also probable, especially early in the wet season. The concentrations of manganese diverge, particularly towards the end of the season. The basin-shaped profile for manganese, particularly at the downstream site, is thought to be due to intrusion of natural groundwater to Magela Creek. The shallow groundwater near the Magela downstream site is known to contain elevated concentrations of soluble manganese. During recessional flow, the groundwater contribution increases as a proportion of total flow, leading to higher values for this element. Although electrical conductivity (EC) is notably higher at the downstream site, this indicator is a less sensitive measure of mine-related contribution than either magnesium or sulfate.

While mine-related changes were evident over the period of monitoring in the 2001-02 wet season, limits were rarely exceeded for any constituent, indicating no environmental harm arising from mine waste water dispersion.

The downstream site at Gulungul Creek has a consistently higher sulfate concentration than the upstream site. This would normally be interpreted as evidence for mine-related impact. However, the concentration of magnesium is not higher at the downstream site whereas elevation would be expected if there was an influence from Ranger mine. This decoupling of magnesium and sulfate suggests that the source of the latter constituent may be the oxidation of sulfide minerals in the catchment, and unrelated to the mine. This interpretation is supported by the generally lower pH at the downstream site, which is the reverse of the observations from Magela and Ngarradj Creeks (and some tributaries of Ngarradj Creek, not shown). Sulfide oxidation is accompanied by a decrease in pH. Gulungul Creek flows through black soils and associated swamps in the vicinity of Ranger, and these may be a source of low levels of sulfide minerals and some heavy metals. The downstream site shows elevated (though not environmentally significant) concentrations of copper and possibly lead. Charts for these metals are shown below. These (particularly copper) are not likely to be attributable to the mine, and strengthen the hypothesis that Gulungul Creek receives a measurable indicator input from riparian soils. Gulungul downstream also has a higher uranium concentration, compared with the upstream site. There is no clear indication of whether this is mine-related or not However, the uranium limit was not exceeded at any time indicating no environmental harm arising from the elevated levels.

The indicators measured for Ngarradj Creek included pH, conductivity, turbidity, uranium, magnesium, manganese, sulfate and calcium. For all indicators, concordance in concentration values was maintained between the upstream and downstream sites throughout the monitoring season. In no case did values measured at the sites diverge in any significant way. These observations allow a high degree of confidence that no measurable mine-related additions of target indicators were observed during the 2001-02 wet season.

The most evident difference between the upstream and downstream sites at Ngarradj Creek is the higher sulfate concentration at the upstream site. This observation is consistent over the wet season and mirrors the baseline data of previous years. The reason is that the west branch of Ngarradj Creek (not routinely sampled), which joins the creek between the two monitoring sites contains lower sulfate than the main branch. This difference is natural and may be related to spatial differences in the previous estuarine character of this region. This consistent reversal of the typical concentration pattern for sulfate (upstream indicator concentrations are usually lower than downstream ones) provides a very sensitive indication of whether mine-related sulfate is entering Ngarradj Creek.

The only other significant divergence between the two sites relates to magnesium. A basin-shaped profile is evident for magnesium measured at both sites, but particularly downstream. Basin-shaped profiles are usually an indication of groundwater intrusion. Although this effect has been observed in previous years (the baseline period), it was more evident for the 2001-02 monitoring season. This may be due to the 2001-02 wet season having below average rainfall, but also a grossly abnormal distribution, where most precipitation fell in January and February. Under these circumstances surface flow will decline much earlier than usual (Ngarradj Creek stopped flowing in early May 2002), but groundwater flows may be much less affected. In this case, magnesium-charged groundwater will make a greater contribution to surface flow, and earlier in the season.